1. Field of the Invention
The present invention relates to a laser processing method for a wafer such that a laser beam is applied to a work surface of the wafer along a separation line formed on the work surface to thereby form a laser processed groove along the separation line on the work surface.
2. Description of the Related Art
As well known in the art, a semiconductor wafer having a plurality of semiconductor chips such as ICs (Integrated Circuit) and LSIs (Large Scale Integration) arranged in a matrix on the upper surface of a semiconductor substrate such as a silicon substrate is fabricated in a semiconductor device fabrication process. In such a semiconductor wafer, the semiconductor chips are separated from each other by a plurality of separation lines called streets preliminarily formed on the semiconductor substrate, and the semiconductor wafer is cut along these streets to obtain the individual semiconductor chips. Further, in an optical device wafer, the upper surface of a sapphire substrate or the like is partitioned into a plurality of regions by a plurality of streets formed in a matrix, and a gallium nitride compound semiconductor or the like is stacked in each of these partitioned regions of the substrate to thereby form an optical device. Such an optical device wafer is cut along the streets into a plurality of optical devices such as light emitting diodes and laser diodes, which are widely used for electrical equipment.
Cutting of a wafer such as a semiconductor wafer and an optical device wafer along the streets is usually performed by using a cutting device called a dicer. This cutting device includes a chuck table for holding a wafer as a work, cutting means for cutting the wafer held on the chuck table, and moving means for relatively moving the chuck table and the cutting means. The cutting means includes a rotary spindle adapted to be rotated at high speeds and a cutting blade mounted to the rotary spindle. In cutting the wafer by using such a cutting device, there is a limit to a feed speed and the devices on the wafer may be contaminated by chips generated in cutting.
As a method for separating a plate-like work such as a semiconductor wafer into a plurality of semiconductor chips or the like, there has recently been proposed a method including the steps of applying a pulsed laser beam to the work along a plurality of separation lines formed on the work to thereby form a plurality of laser processed grooves by ablation and next breaking the work along these laser processed grooves by using a mechanical breaking device (see Japanese Patent Laid-open No. 10-305420, for example).
Such laser processing has advantages over cutting such that a processing speed is higher and a wafer formed of a hard material such as sapphire can be processed relatively easily. However, in performing the ablation by the application of a laser beam to the wafer along the separation lines, thermal energy is concentrated at a region irradiated with the laser beam, causing the generation of debris, and this debris may stick to the surfaces of the chips formed on the wafer, causing a degradation in quality of the chips.
To solve this problem due to the debris, there has recently been proposed a laser processing method including the steps of coating the work surface of a wafer with a protective film formed of liquid resin such as polyvinyl alcohol and next applying a laser beam through the protective film to the work surface of the wafer (see Japanese Patent Laid-open No. 2004-188475, for example). Further, there has been proposed a laser processing device including protective film forming means for forming a protective film of liquid resin such as polyvinyl alcohol on the work surface of a wafer, so as to efficiently perform laser processing (see Japanese Patent Laid-open No. 2004-322168, for example).
The protective film of liquid resin such as polyvinyl alcohol as formed on the work surface of the wafer is solidified at ordinary temperatures. Accordingly, the laser beam is applied through the solid protective film to the work surface of the wafer to form the laser processed grooves by ablation, so that the protective film is separated from the work surface of the wafer, causing the deposition of debris on the work surface of the wafer on both sides of each laser processed groove. Further, in the case that the debris that has scattered falls onto the protective film and that the debris is relatively large, the solid protective film is melted by the heat of the debris, so that the debris comes into direct deposition onto the surface of any device formed on the wafer, causing a degradation in quality of the device.
Further, the protective film is absent in a region where the laser processed grooves have been formed along the separation lines extending in a first direction. Accordingly, in forming the laser processed grooves along the separation lines extending in a second direction intersecting the first direction, the debris that has scattered comes into deposition to the wall surface of each laser processed groove already formed. The deposition of the debris to the wall surface of each laser processed groove causes a problem that the wafer cannot be reliably broken along each laser processed groove in the next breaking step.